Basically, forwards means better turning (looser) and further back means more stable. But in practice, it is very muh a question of tuning the balance of the board to suite your style and sails. So my advice is that if you're a bit lazy, just leave it in the middle. If you're eager to learn, try it all the way in front, then all the way back and then, when you have a feel for what the adjustments really do, try fine tuning with some settings in between.